Influenza virus subtypes that bind to platelet sialoglycans. (A) Fixed, washed human platelets attached to a BLI sensor and visualized by light microscopy (original magnification ×20). Red arrow: single platelet. (B) Determination of mean virus particle count by NTA. Total peak values are presented (n = 5). A/H3N2 (5.4 × 10¹⁰ ± 3.6 × 10⁹ virus particles per milliliter); A/H1N1 (3.6 × 10¹⁰ ± 9.4 × 10⁸ virus particles per milliliter); and A/H5N1 (2.2 × 10¹⁰ ± 1.8 × 10⁹ virus particles per milliliter). (C) A highly concentrated A/PR/8/34 (PR8; 1 × 10¹² virus particles per milliliter) virus was used for a full titration on fixed platelets in the presence of OSC (100 µM). Mean (n = 2); error bars, SD. The K_D value for PR8 (9.5 ± 1.0 pM) was determined from a Langmuir model. The dotted line represents the plateau value (b = 17.89 nm). (D) Enlargement of the dotted box in panel C. Estimated plateau values (b) are 3.91 nm (95% CI, 3.41-4.53 nm) for A/H3N2, 10.6 nm (95% CI, 9.33-12.13 nm) for A/H1N1, and 12.7 nm (95% CI, 11.43-14.20 nm) for A/H5N1. (E) Virus binding to α2,6-sialoglycan density gradients ranging from 0 to 11.75 pmol/cm². Obtained threshold glycan densities are given above each curve. (F) The same as in panel E, but for the α2,3-sialoglycan density gradient. (G) Virus association and dissociation to BLI-sensors functionalized with α2,3-sialoglycans in the presence and absence of OSC (100 µM).